N-heterocyclic carbenes:: Exotic molecules as precursors to unusual hydrogen bonds

Hydrogen bonds are weak directional forces that are evident in a large number of chemical systems. Although they have been studied for over a century, the range of donor-acceptor pairs continues to be extended. Recently, isolable N -heterocyclic carbenes have been prepared and these molecules provide a novel route into a number of unusual hydrogen bonds. Here we briefly review structural studies on these interactions.